PPARdelta is a member of the PPAR family of nuclear receptors and is ubiquitously expressed. Activation of PPARdelta promotes fat burning. Highly specific synthetic PPARdelta ligands (agonists), such as GW501516 (GW), are promising drug candidates for obesity and diabetes. Endogenous PPARdelta is abundantly expressed in mouse embryonic fibroblasts (MEFs) but associates with histone deacetylases and behaves as a transcriptional repressor in the absence of ligand. Upon ligand treatment, endogenous PPARdelta switches from a repressor to an activator, which leads to a robust activation of target genes such as Angptl4. We are using the GW-induced Angptl4 expression in MEFs as a model system to investigate the roles of histone acetyltransferases GCN5/PCAF, CBP/p300, and associated histone acetylations in regulating expression of endogenous nuclear receptor target genes. We report that the two pairs of histone acetyltransferases (HATs), GCN5/PCAF and CBP/p300, are specifically required for H3K9 acetylation (H3K9ac) and H3K18/27 acetylation (H3K18/27ac), respectively, in cells. Further, we show that CBP/p300 and their HAT activities are essential, while GCN5/PCAF and associated H3K9ac are dispensable, for ligand-induced nuclear receptor target gene expression. These results highlight the substrate and site specificities of HATs in cells, demonstrate the distinct roles of GCN5/PCAF- and CBP/p300-mediated histone acetylations in gene activation, and suggest an important role of CBP/p300-mediated H3K18/27ac in nuclear receptor target gene expression (Jin Q. et al., EMBO J, 2011). Using ligand-induced PPARdelta target gene expression in MEFs as a robust model system, we will continue to study epigenetic regulation of NR target gene expression. The results will be verified using other NRs including PPARgamma. 1st, we will use MEFs derived from our collection of conditional KO mice to investigate the roles of site-specific histone methylations in NR target gene expression. 2nd, we will investigate how CBP/p300-mediated H3K18/27ac regulate NR target gene expression. We hypothesize that CBP/p300-mediated H3K18/27ac may be recognized by yet to be identified effector proteins, which recruit Pol II to initiate transcription. We will use affinity purification to isolate and determine the identities of the effector proteins. 3rd, we will investigate the roles of other epigenetic mechanisms, such as chromatin remodeling, and their functional cooperation with histone modifications, in ligand-induced NR target gene expression.